This invention relates to seals for rotating shafts, and more particularly to seal assemblies especially adapted for sealing glass coated agitator shafts to mixer tanks handling corrosive materials.
FIG. 1 and U.S. Pat. No. 3,877,706 illustrate seals which have been developed for mixers handling corrosive materials. Many such mixers have glass lined tanks and glass coated agitator shafts to resist corrosive attack. Since the gap between the agitator shaft and the opening for it into the tank must be closed, especially when mixing is done under pressure, a rotatable carbon seal is commonly used which moves with the agitator shaft and slides and seals against the polished face of a substantially non-metallic (e.g., ceramic) stationary seal. Prior art seals using this approach have satisfactorily closed and sealed the gap between the agitator shaft and the mixer tank without exposing metallic seal members to corrosive materials within the tank.
Generally, however, such prior art devices have been of the type requiring assembly directly onto the agitator shaft at the application site. Such assembly is a time-consuming process since the seals must be carefully adjusted so that the springs which bias the moving carbon seal against the stationary ceramic seal will provide a proper, uniform sealing pressure. The seal is then pressure tested for leakage, prior to returning to service. During such periods of assembly and adjustment, the mixer is necessarily shut down, sometimes for several days, causing substantial lost production.
"Cartridge" seals have therefore been developed to reduce equipment down time. These permit the seal components to be preassembled and adjusted prior to attachment to the agitator shaft and the mixer. For example, the above-noted U.S. Pat. No. 3,877,706 shows a seal in which the central core, carrying the moving carbon and stationary seal members, may be extracted as a unit from the seal housing by lifting it vertically along and beyond the end of the agitator shaft and slipping a new core down onto the shaft and back into the seal housing.
Fully cartridge bench testable mechanical seals are also available for relatively less corrosive environments, such as the example shown in FIG. 1, wherein the entire seal, including the seal housing, is removable from the shaft by sliding it along and of the end of the shaft. Such a structure is known as a "fully cartridge" seal since the entire unit may be changed as a single, "fully" cartridge system, and it differs in this respect from the construction of U.S. Pat. No. 3,877,706 which is not truly a cartridge seal since only part of the seal is removed for servicing. In order to explain the advantage provided by the present invention over the structure of FIG. 1, the latter will now be described.
In FIG. 1, the seal assembly indicated generally as 10 supports the shaft 11 in depending relation into the tank shown fragmentarily at 13. The seal assembly 10 is supported by an annular base plate 15 which is clamped or otherwise secured on tank 13 in aligned relation with the opening 16 through which the shaft 11 extends. The junction between plate 15 and tank 13 is sealed by a gasket 17 which is preferably provided with a corrosion-resistant jacket 18 of Teflon or the like, and the surfaces of the tank wall 13 and plate 15 exposed to the contents of the tank are shown as having a glass coating or lining G.
The seal assembly 10 includes a sleeve 20 sealed on the outside of shaft 11 by O-rings 22, and the sleeve 20 is proportioned to extend down below the upper end of the glass coating G on the shaft so that the lower O-ring 22 seals against this coating. The upper end of the sleeve 20 is supported by a ball bearing 25 in inner and outer housings 26 and 27 provided with a top cap 28. This bearing assembly is supported by the top plate 30 of the seal housing, which is in turn supported by the cylindrical seal housing 31 and bottom seal housing plate 32 on the plate 15 by means of bolts 33 and gasket 34.
The inner bearing housing 26 has a slip fit relation with sleeve 20 for ready assembly and disassembly purposes, but both it and the sleeve are secured for rotation with shaft 11 by a key 35 held in a slot 36 in shaft 11 and a slot 37 in sleeve 20 by a set screw 38. Sealing rings 39 between housing members 26 and 28 and between sleeve 20 and housing member 27 retain grease within the bearing assembly.
A stationary seal 40 of ceramic or other suitable material is mounted in the lower end of seal housing 31, and a gasket 42 seals the junction between seal 40 and plate 32. A second stationary seal 44 is similarly mounted in the top plate 30 of the seal housing. The seal 44 and the top plate 30 are proportioned to provide free running clearance for the sleeve 20.
The opposed pair of rotating seals which complement stationary seals 40 and 44 are carried by a seal retainer 45 proportioned for slip fitting on the sleeve 20 and secured thereto by one or more set screws 46. Each rotating carbon seal 50 is mounted loosely in one or the other end of retainer 45 with a loosely keyed connection thereto (shown at 151 in FIG. 3), and it is held against accidental removal from retainer 45 by a snap ring 52. Each seal 50 has a frustoconical inner surface mating with the complementary outer surface of a wedge ring 55 of a Teflon or the like to cam the ring 55 into sealing engagement with the sleeve 20.
Each wedge ring 55 is lightly biased by springs 56 through an annular disk 57 and an O-ring 58 to urge the seals 50 to sealing relation with their associated stationary seals 40 and 44. In operation, the space 60 surrounding seal retainer 45 is pressurized with sealant through the inlet 61 to force the seals 50 and wedge rings 55 to sealing positions, the retainer 45 having openings 62 for admitting the pressure fluid to the interior of retainer 45 to exert pressure on the seals and wedge rings.
The fully cartridge seal of FIG. 1 can be mounted on sleeve 20 and adjusted and bench tested prior to assembly on the agitator shaft by pressurizing the seal housing and checking for leakage. Seal assemblies passing this test can then be stored until needed, and can then be put into service quickly as a unit replacing the seal and bearing assembly which is removed from service. The boss 65 on shaft 11 is used during replacement of the seal assembly for temporarily supporting the shaft and agitator within the tank, in known manner.
The fully cartridge seal shown in FIG. 1 offers substantial advantages over other prior art constructions such as that shown in U.S. Pat. No. 3,877,706, but it hasone potential weakness in that the nose portion 66 at the lower end of sleeve 20 is exposed to the contents of the tank 13, which is undesirable if the contents are corrosive. Attempts have been made to overcome this potential problem by coating the nose of the sleeve with glass, but this is expensive and also requires that the relatively thin and flexible sleeve be handled with care to avoid cracking the glass coating.
Another solution to the problem of sleeve corrosion is provided in U.S. Pat. No. 3,877,706, wherein the sleeve is effectively shifted to a position outside the seal members, but this has disadvantages. Because interior sleeves have become generally standard in the industry, standard sized carbon seal units for use therewith are available from many manufacturers, and special seal assemblies are required if some other sleeve arrangement is used. Further, an interior sleeve, as in FIG. 1, can support the seal during bench testing, but if an exterior sleeve is used, the sleeve must be supported by the agitator shaft when the sealant space is pressurized for testing, and the seal must therefore be disassembled after testing and cannot be put in service as a preassembled unit.
A need therefore remains for a fully cartridge, fully bench testable, corrosion resistant mechanical seal particularly adapted for rotatably sealing a glass lined agitator shaft to a mixer tank and which exposes only non-metallic parts to the potentially corrosive contents of the tank. Such a seal would provide maximum service life, maximum resistance against attack and deterioration, maximum convenience in preparation and assembly of the seal, maximum convenience in assembly onto the agitator shaft and mixer tank, and minimum loss of production time.